Fiber-optic telecommunications systems including a laser diode, an external modulator and a photodetector diode are well-known in the field for transmitting optical signals over optical fiber or similar optical waveguides. Chromatic dispersion in optical fiber tends to make the achievable transmission distance of fiber optic communications systems dependent on the modulation rate and the modulation chirp parameter. External modulators, when used to modulate the continuous wave (CW) input optical power from the laser, permit the modulation chirp parameter to be adjusted to a substantially fixed value in a predetermined, controllable manner, thereby minimising the transmission power penalty caused by chromatic dispersion.
External modulation is accomplished, for example, in a dual waveguide device wherein substantially identical input optical beams are supplied to the waveguides and wherein each waveguide is subject to its own individual control. Modulation signals are applied to each waveguide via the separate control. Moreover, control signals are applied to each waveguide for adjusting the modulation chirp parameter to a desired non-zero substantially fixed value.
Typical high-speed electro-optical external modulators use a traveling-wave electrode structure to form a microwave transmission line in the vicinity of the optical waveguide. A microwave signal co-propagates with an optical signal for a prescribed distance, thereby achieving the required optical modulation. To prevent velocity mismatch between the microwave signal and the optical signal in a traveling wave modulator, a thick buffer layer is provided on a wafer to speed up the propagation of the microwave signal. More specifically, the thick buffer layer provides a substantially dielectric medium through which the microwave energy can be distributed. Previously, a silicon dioxide (SiO2) buffer layer was created through known techniques such as electron beam, sputtering, or chemical vapor deposition (CVD). The buffer layer may be planarized throughout the wafer or may be patterned with electrode structures.
Using a SiO2 buffer layer has numerous advantages. A SiO2 buffer layer is produced by devices such as evaporators, sputtering machines, gas supply machines or CVD machines which permit very precise control layer thickness and/or composition. Both of these parameters influence the velocity of propagation of the electrical RF signal as well as the optical signal in the waveguide.
For many applications such as high-speed telecommunications systems, it is important to achieve a high modulation efficiency, which is generally measured in terms of the magnitude voltage Vπ (sometimes denoted Vpi) which needs to be applied to the modulator electrodes to achieve an optical phase shift of π (pi). Typical design targets are 5 volts or less, however this may vary from manufacturer to manufacturer. Lithium niobate (LiNbO3) is an electro-optic material which can meet this design criterion.
Lithium niobate (LiNbO3) is used in two main crystallographic orientations: X-cut and Z-cut. The term X-cut or Z-cut LiNbO3 refers to LiNbO3 that is cut perpendicular to the X- or Z-crystallographic orientation respectively. The Y-cut is crystallographically equivalent to X-cut and therefore is covered in all descriptions where X-cut is treated.
Most applications require very stable operation of the electro-optical modulator over time and through changes in temperature, humidity and other environmental conditions. In other words, the operating (bias) point of the modulator should remain constant. The operating point of the modulator is controlled via a low frequency or DC bias voltage. The high-frequency modulation signal may be superimposed on the low frequency or DC bias voltage and applied to the RF electrodes, or may be applied to separate bias electrodes.
LiNbO3 is sensitive to temperature changes because the pyroelectric effect in LiNbO3 creates mobile charge as a result of temperature changes within the device. The mobile charges can generate strong electric fields during normal operation of the device. Such strong electric fields are problematic because they can change the operating (bias) point of an electro-optic modulator, such as a Mach-Zehnder Interferometer (MZI), by creating fields across the waveguides that do not match one another. In addition, these strong electric fields can cause time dependent or uncontrolled charge dissipation, which may result in a loss of transmitted data. These fields may also cause arcing, which may also result in a loss of transmitted data.
There are methods known in the art for bleeding off pyroelectric charge. For example, in Z-cut substrates the pyroelectrically generated electric fields are generated in a direction perpendicular to the modulator plane. Some prior art devices use a metal oxide or semiconductor layer that is formed on top of the thick buffer layer to bleed off pyroelectric charge through a conductive path to the bottom of the device. Both amorphous and polycrystalline-silicon (poly-Si) semiconductor layers have been used to bleed off pyroelectric charge. A diffusion-suppressing layer is sometimes included to prevent the metal electrodes from diffusing into the semiconductor bleed-off layer.
Other prior art devices use a conductive layer on the bottom of the device that is electrically connected with the ground electrodes to provide a discharge path. In these devices, charge accumulating on the hot electrode can find a path to ground through the driver or biasing electronics.
A problem associated with LiNbO3 modulators is the charge generation and charge redistribution that can occur in the buffer when a bias voltage is applied to an electrical input of a LiNbO3 Mach-Zehnder interferometric modulator. More specifically, the bias voltage, applied to control the operating point of the Mach-Zehnder interferometer, can cause the formation of mobile charges, in the form of either electrons, holes, or ions in the buffer. These mobile charges either counteract the effect of the applied voltage by establishing a positive DC drift, or enhance the applied bias voltage by establishing a negative DC drift. Positive drift is particularly problematic because the voltage required to maintain the bias condition will steadily increase (“run away”) causing a control system reset to occur, which will result in loss of data. There are methods known in the art for reducing DC drift.
Prior art designs in U.S. Pat. Nos. 5,404,412 and 5,680,497 reduce the effect of the buffer layer charging by doping the buffer layer, causing it to be more conductive. The added conductivity in essence shorts out the buffer layer, preventing the buffer layer from charging up. Accordingly, a slowly varying voltage applied to the gold electrodes is able to control the bias point of a Mach-Zehnder Interferometer over time. Alternatively, designs for x-cut lithium niobate may have a separate electrically isolated low frequency bias electrode, optically in series with the RF electrode. This separate bias electrode does not have a buffer layer between the electrode and substrate, eliminating problems associated with the buffer layer, however it does increase the length of the device.
Designs for z-cut lithium niobate with separate bias electrodes are shown in U.S. Pat. No. 5,359,449. Z-cut lithium niobate electrode designs (bias or RF) typically require a buffer layer, as the electrodes must always be positioned over the waveguide. In some prior art lithium niobate designs, bias control is achieved with a separate bias electrode made of an optically transparent conductor, such as Indium Tin Oxide (ITO), placed on top of the waveguide.
Note that typically the entire device is usually placed in a hermetic package to prevent moisture from reaching the electrodes.
U.S. Pat. Nos. 5,895,742 and 6,198,855 B1 discuss designs using polymer buffer layers. The U.S. Pat. No. 6,198,855 B1 describes a z-cut device with a conductive or non-conductive buffer layer, with a bleed layer formed on top of the buffer layer, or directly on the surface. Note however that the bleed layer material is not patterned to form electrodes, nor does it provide a means to externally control the electric potential in the vicinity of the waveguides.
U.S. Pat. Nos. 6,195,191 B1 and 6,282,356 B1 describe means of treating the surface of the substrate to change conductivity or to reduce surface damage to improve bias stability. The use of bleed layers is also described. Note that the entire surface is treated. No attempt to create electrodes with the surface treatment is discussed.
Other prior art includes U.S. Pat. No. 5,214,724, where a semiconductive electrode is placed laterally next to the main signal electrodes. Note that all electrodes are on top of the buffer layer, in contrast to the invention described here, where the bias electrodes reside on the surface of the substrate. U.S. Pat. No. 5,214,724 teaches that a semiconductive electrode can be used for low frequency control of the bias point. Note that the claims also include a bleed layer, called a “primary semiconductive layer,” between all the electrodes and the buffer layer.
Japanese patent 1789177 (grant date Sep. 29, 1993) describes a patterned buffer layer with a semiconductive bleed layer over top of the patterned buffer layer and on top of the surface of the substrate, in regions where there is no buffer layer.
In U.S. Pat. No. 6,853,757, a transparent conductive film underneath a highly conductive metal electrode applies a voltage directly to the surface of the substrate. The metal electrode is shifted laterally with respect to the center of the waveguide to minimize optical loss. Note that the transparent conductive film is intended to carry both high and low frequency signals from the highly conductive electrode to the waveguide. As stated in the patent application, “the invention is particularly advantageous since it becomes possible to prevent optical loss and to achieve further high-speed modulation by forming a metal electrode so that the metal electrode may not be superimposed as much as possible on a part formed on an optical waveguide in a transparent electrode.”
U.S. Pat. No. 5,455,876 describes a design with highly conductive (preferably gold) electrodes on the surface of the substrate and underneath the buffer, but with a floating electrical potential. The floating electrodes are DC isolated from the electrodes on top of the buffer and have no external DC connection. The floating electrodes are intended to improve high frequency modulation efficiency by capacitively coupling RF from the electrode on top of the buffer. Their proximity to the electrode results in efficient modulation for the fraction of voltage that is coupled. In a journal article by Samuel Hopfer, et. al., entitled “A novel wideband, lithium niobate electrooptic modulator,” in the Journal of Lightwave Technology, Vol. 16, No. 1, January 1998, pp. 73-77, the inventor states that the purpose of the floating electrodes is “for the purpose of applying the available RF voltage directly across the titanium indiffused optical waveguides.” Note that the floating electrodes do not provide any mitigation of the bias voltage drift due to the buffer charging effect, since they lack the external DC connection.
U.S. Pat. No. 6,310,700 is somewhat similar to U.S. Pat. No. 5,455,876, in that there is a set of large electrodes on top of a buffer layer, and a set of electrodes on the surface of the substrate. Instead of relying on capacitive coupling of the signal voltage from the upper to lower electrodes, conductive legs connect the two sets of electrodes. Note that the bottom set of electrodes are directly interconnected with the upper electrodes at both high and low frequencies. They are intended to carry the voltage from the top electrode to the bottom set of electrodes for all frequencies. The key feature to note is that the modulation is produced by the lower set of electrodes at high and low frequencies. The patent states, “the thickness of the buffer layer 400 should be thick enough such that the electric field 710 generated by the electrical signals propagating in the transmission line 300 does not reach the lithium niobate substrate slowing down the electrical velocity.” If the field lines from the transmission line do not reach the substrate, then those field lines play a minimal role in modulation at both high and low frequencies. Furthermore, the patent teaches, “in particular, the conductive legs 350 must be long enough to elevate the transmission line 300 away from the substrate 100 such that the stronger parts of the electric field generated by the electrical signals propagating in the transmission line 300 (hereinafter the “electric field of propagation 710”) does not reach the lithium niobate substrate 100 slowing down the electrical velocity. The electric field of propagation 710 (shown in FIG. 3) is generated across the gaps between the electrodes of the transmission line 300, but does not perform the modulation of the optical signals.” Hence, the modulation at high and low frequencies is performed by the set of electrodes on the surface of the substrate, referred to as a “loading electrode.” The patent also states that “the opposing loading electrodes of the opposing conductive legs generate a capacitance that reduces the electrical velocity on the transmission line to match the optical velocity of the optical signal,” hence, the loading electrodes are strongly coupled to the transmission line at high frequency.